Clefts of lip and palate are among the most common birth defects, occurring in approximately every 700 live births. Although treatable by reconstructive surgery, oral clefts cause complications early in life ranging from feeding problems to hearing and speech defects. In addition, many of these patients also show mental retardation or other neurodevelopmental problems. In a forward genetic screen carried out in mice we recovered a novel line named manta-ray (mray), which displays severe developmental defects including a median cleft of the upper jaw and secondary palate as well as abnormal brain development. We identified the causative recessive mutation in the Pak1ip1 gene, which encodes a preribosomal factor required for ribosome biogenesis. Loss of Pak1ip1 activates the tumor suppressor and cell cycle control factor Tp53, which leads to cell cycle arrest and cell death. This pathway appears to be a critical mediator of many of the clinical features of ribosomopathies including their craniofacial anomalies. Our current understanding of Pak1ip1 function strongly supports the hypothesis that Pak1ip1- deficiency in homozygous mray mutants activates the Tp53 pathway and leads to Tp53-mediated loss of cranial neural crest cells, which results in midline clefting. The experiments we propose here, will uncover the mechanisms that cause the craniofacial anomalies of homozygous mray mutants by providing a detailed analysis of neuroepithelial cell death, proliferation, and specification of neural crest-derived tissues. In second set of experiments, we will attempt to ameliorate or even rescue the midline cleft in affected mutants by inhibiting Tp53 response either through pharmacological or genetic means. Finally, we will examine the way the mray mutation affects the process of ribosome biogenesis in general and in particular, by analyzing specific deficits in the translation of midface-specific growth factors. In summary, our study will provide substantial new insight into the mechanisms by which defects in ribosome biogenesis can lead to craniofacial birth defects, which are common and clinically important and will explore treatment options by interfering with the Tp53 pathway.